Hoshanah Rabbah:

The Nature of the Day

Hoshana Rabbah marks the day when the day of judgment, which begins on Rosh Hashanah, is sealed. At the beginning of the period of judgment - on Rosh Hashanah and Yom Kippur - all the world's inhabitants pass in individual review before G-d. During the Festival of Sukkot, the entire world is judged concerning water, fruit, and produce.

The seventh day of the Festival, Hoshana Rabbah, is the day on which this judgment is sealed. Because human life depends on water and all depends upon the final decision, Hoshana Rabbah is invested with a certain similarity to Yom Kippur and is therefore marked by profuse prayer and repentance.